1. Field of the Invention
The present invention relates to hologram recording apparatuses and/or hologram reconstructing apparatuses.
2. Description of the Related Art
A hologram memory, serving as a data storage device, has been a focus of attention in recent years. In the hologram memory, a hologram recording apparatus is used to record a hologram, while a hologram reconstructing apparatus is used to reconstruct the recorded hologram. Hologram recording is performed as follows. That is, a signal beam modulated according to data to be recorded and a predetermined reference beam are generated from laser light emitted from the same light source and applied to a hologram recording medium, in which the signal beam and the reference beam interfere with each other to form an interference pattern (hologram). Thus, the data is recorded on the hologram recording medium as a hologram. The recorded hologram contains a large amount of information recorded in a unit called a page. The recorded data is identified and managed on a page-by-page basis.
In this hologram memory, the hologram reconstructing apparatus is used to reconstruct the recorded data from the hologram recording medium. Hologram reconstruction is performed as follows. That is, a reconstruction beam that is a light beam having characteristics identical to those of the reference beam used in recording is applied to the hologram formed according to the data described above. This causes a diffracted beam to emerge from the hologram recording medium. The diffracted beam, which contains a page of recorded data, is detected by a two-dimensional array of photodetectors and subjected to signal processing. Thus the recorded data can be reconstructed.
Besides, there has been proposed a hologram recording/reconstructing apparatus (recording and reconstructing apparatus) that is capable of performing the functions of both the hologram recording apparatus and the hologram reconstructing apparatus. In the following description, the term “hologram recording/reconstructing apparatus (recording and/or reconstructing apparatus)” is used to collectively refer to a hologram recording apparatus, a hologram reconstructing apparatus, and a hologram recording/reconstructing apparatus, and if a clear distinction is needed, it will be described accordingly. Similarly, the term “recording/reconstruction (recording and/or reconstruction)” is used to collectively refer to recording, reconstruction, and recording/reconstruction (recording and reconstruction), and if a clear distinction is needed, it will be described accordingly.
Generation of the signal beam, reference beam, and reconstruction beam and detection of the diffracted beam are performed in a hologram recording/reconstructing optical section formed by combining optical elements. Examples of methods for designing an optical path in the optical section include a so-called coaxial method (see, e.g., Nikkei Electronics, Jan. 17, 2005, pp. 106 to 114) in which the signal beam and the reference beam are coaxially arranged, the paths of the reference beam and reconstruction beam partially overlap, and these optical beams (signal beam, reference beam, and reconstruction beam) pass through a common optical path. Another method for designing an optical path in the optical section is a two-beam method in which the signal beam and the reference beam (reconstruction beam) pass through different optical paths.
In the hologram memory, if a temperature during recording and a temperature during reconstruction are different from each other by several ° C., it is difficult to reconstruct the recorded data due to thermal expansion or contraction of the hologram recording medium. FIG. 17A to FIG. 17C schematically illustrate thermal expansion and contraction of a hologram recording medium. Specifically, FIG. 17A to FIG. 17C each illustrate a hologram corresponding to a given temperature of the hologram recording medium. For ease of explanation, only a substrate 48a, a recording material layer 48b, and a substrate 48c of the hologram recording medium are shown, and the illustration of a reflective film and the like is omitted. As illustrated, the recording material layer 48b is interposed between the substrate 48a and the substrate 48c. In the following description, the term “thermal expansion” is used in a broader sense and includes thermal contraction, but both the terms “thermal expansion” and “thermal contraction” will be used if a clear distinction therebetween is necessary.
When the temperature changes, the hologram recording medium is thermally expanded in a Z-direction (see the lower left part of FIG. 17C) under the influence of a thermal expansion coefficient of the recording material layer 48b (typically about 5×10−4/° C.). In an X-direction and a Y-direction (see the lower left part of FIG. 17C), the intervals and directions of recorded interference fringes of the hologram are changed by pressure from the substrates 48a and 48c which are hard and have a small thermal expansion coefficient (about 7×10−6/° C. to 8×10−6/° C.). FIG. 17B schematically illustrates a state of a hologram when the temperature of the hologram recording medium is 25° C. (reference temperature). In FIG. 17B, L1 denotes a thickness of the recording material layer 48b, d1 denotes a distance between adjacent fringes, and α1 denotes an inclination of fringes with respect a surface of the hologram recording medium.
If the temperature of the hologram recording medium becomes 15° C., the hologram recorded at a temperature of 25° C. (reference temperature) is changed to the state illustrated in FIG. 17A. In FIG. 17A, L2 denotes a thickness of the recording material layer 48b, d2 denotes a distance between adjacent fringes, and α2 denotes an inclination of fringes with respect a surface of the hologram recording medium. Due to thermal contraction of the recording material layer 48b, the thickness L2 becomes smaller than the thickness L1, the distance d2 becomes smaller than the distance d1, and the inclination α2 becomes greater than the inclination α1. If the temperature of the hologram recording medium becomes 35° C., the hologram recorded at a temperature of 25° C. (reference temperature) is changed to the state illustrated in FIG. 17C. In FIG. 17C, L3 denotes a thickness of the recording material layer 48b, d3 denotes a distance between adjacent fringes, and α3 denotes an inclination of fringes with respect a surface of the hologram recording medium. Due to thermal expansion of the recording material layer 48b, the thickness L3 becomes greater than the thickness L1, the distance d3 becomes greater than the distance d1, and the inclination α3 becomes smaller than the inclination α1.
As described above, if there is a difference between the temperature of the hologram recording medium during recording and that during reconstruction, the shape of the hologram is changed accordingly. As a result, when a light beam having the same wavelength and incident direction as those of the reference beam used during recording is used as a reconstruction beam for obtaining a diffracted beam from the hologram recording medium to reconstruct the recorded data, since a Bragg condition of interference fringes is not satisfied and no diffracted beam is generated, it is difficult to reconstruct the recorded data from the diffracted beam. To solve this, the inventor listed in the present application proposed a method for changing the wavelength and incident direction of the reconstruction beam from those of the reference beam, and also proposed a tunable (wavelength variable) laser for varying the wavelength (see, e.g., T. Tanaka, K. Sako, R. Kasegawa, M. Toishi, K. Watanabe, and S. Akao, “Tunable blue laser for holographic data storage” Proceedings of Optical Data Storage, 2006, pp. 215 to 217). As for the incident direction, it is easy, with the two-beam method described above, to change the incident direction of either of the reconstruction beam and the reference beam.